As lithographic offset printing plates, PS (presensitized) plates and the like, using a positive light sensitizer containing a diazo compound and a phenolic resin as main components, or a negative light sensitizer containing an acryl monomer or prepolymer as a main component, are now in practical use. However, since these printing plats are of low sensitivity, platemaking is conducted by bringing a silver salt photographic film original plate previously imagewise recorded into initimate contact and then exposing to light.
With computer image processing, storage of large volumes of data, and advances of data communication technology, an electronic editing system in which input of an original, correction, editing, layout and further paging are totally processed with a computer, and which permits output to terminal plotters at a remote plate by the use of a high speed communication network or a satellite communication has now come into practical use. This electronic editing system is highly desirable in the field of newspaper printing needing instantaneous communication. Also, in fields in which an original is stored as an original film, and, based on this film, a printing plate is copied, if necessary, with the advance of development of super high volume recording media such as a light disk, it is considered that the original is stored as digital data in the recording media.
A direct printing plate in which a printing plate is produced directly from an output of a terminal plotter has not almost been put into practical use. Even in fields in which the electronic editing system works, data are output to a silver salt photographic film, and, based on this, intimate contacting and exposure to light are applied indirectly to a PS plate to produce a printing plate. This is due to the fact that it is difficult to develop a direct printing plate having a sufficiently high sensitivity to permit production of a printing plate within a practially allowable time by the use of a light source of an output plotter (e.g., He-Ne laser or semiconductor laser).
An electrophotographic light-sensitive material can be considered as a light-sensitive material having a high light-sensitivity capable of providing a direct printing plate.
As printing plate materials (printing original plates) utilizing electrophotography, for example, zinc oxide-resin dispersion type offset printing plate materials as described in JP-B-47-47610, JP-B-48-40002, JP-B-48-18325, JP-B-51-15766 and JP-B-51-25761 (the term "JP-B" as used herein means an "examined Japanese patent publication") have heretofore been known. These materials are used, after forming a toner image by the electrophotographic method and then wetting with a solution to make non-image areas oil-desensitive (e.g., an acidic aqueous solution containing a ferrocyanide or a ferricyanide). An offset printing plate subjected to such treatment possesses printing durability such that about 50,000 to 100,000 sheets can be printed, and is unsuitable for printing more than 100,000 sheets. It also has disadvantages, in that if the composition is changed so as to be suitable for the treatment, electrostatic characteristics are deteriorated and image quality is reduced. Furthermore, the solution uses a toxic cyanic compound.
Inorganic photoconductive material-resin based printing plate materials as described in JP-B-37-17162, JP-B-38-7758, JP-B-46-39404 and JP-B-52-2437, for example, an electrophotographic light-sensitive material produced by providing a photoconductive insulating layer in which oxazole or an oxadiazole or an oxadiazole compound is bound with a styrene-maleic anhydride copolymer, on a grained aluminum plate is used. On this light-sensitive material, a toner image is formed by the electrophotographic process and then non-image areas are removed with an alkaline organic solvent to produce a printing plate.
JP-A-57-147656 (the term "JP-A" as used herein means an unexamined published Japanese patent application) discloses an electrophotographic light-sensitive material containing a hydrazone compound and barbituric acid or thiobarbituric acid. In addition, JP-A-59-147335, JP-A-59-152456, JP-A-59-168462 and JP-A-58-145495 disclose an electrophotographic light-sensitive material in which dye is sensitized. However, in non-image areas of a printing plate produced using the above electrophotographic light-sensitive material, a substance in the electrophotographic light-sensitive layer is adsorbed, leading to contamination of the nonimage areas. Due to the attachment of ink to the nonimage areas of prints and staining, the printing plate is not satisfactory for use as a printing plate. U.S. Pat. No. 3,181,461 describes that a so-called PS plate obtained by treating an anodized aluminum plate with an alkali metal silicate solution, and coating a light-sensitive layer exhibits good resistance against contamination of non-image areas. However, a printing plate using an electrophotographic light-sensitive material in which a photoconductive layer is provided on a substrate subjected to surface treatment with an alkali metal silicate generally has poor printing durability and is unsuitable for practical use.
In the production of a lithographic printing plate, a so-called gum solution is coated in the final step.
This gum solution coating is applied not only for the purpose of protecting hydrophilic properties of non-image areas but also for the purpose of protecting against contamination or scratching due to attachment of finger prints, oils and dust during storage until image areas are written or removed, or until printing is conducted after plate making, or until reuse, or during the attachment or handing of a printing machine. Further it is applied for the purpose of preventing contamination of the surface due to oxidation.
As a gum solution for a lithographic printing plate, an aqueous solution of gum arabic, cellulose gum or a water-soluble polymeric substance has heretofore been used. This gum solution, however, when a large amount of an organic contamination substance is adsorbed thereon, is poor in an oil-desensitization capability and is contaminated.
When contamination due to the above causes is generated, a step of removing organic contaminating substances is needed. A contamination removing solution to be used for the above purpose is commercially available (for example, Plate Cleaner CU-3 produced by Fuji Photo Film Co., Ltd.). Such an additional step is undesirable because of a reduction in workability.